1. Field of the Invention
The invention relates to borehole geophysical measurement and oil field development technology, and more particularly to a borehole electromagnetic detection method to acquire outer cased well formation resistivity distribution information by means of borehole excitation and observation of electromagnetic field changes to evaluate remaining oil distribution in oil reservoir development.
2. Description of the Related Art
Because electrical parameters of a reservoir medium are closely related to reservoir porosity, pore fluid property, and saturation, the electrical parameters when the reservoir contains oil gas have a huge difference from those when the reservoir contains formation water. The electrical parameter of the reservoir medium is a major parameter to determine and evaluate oil-bearing property of the reservoir. Thus, electrical or electromagnetic methods have always been important to realize quantitative evaluation of reservoir oil-bearing property. As it can be done directly in wells to measure in the middle or near the middle of a reservoir, well logging is one of the geophysical detection methods with a highest resolution.
Electrical logging used for the borehole formation resistivity measurement has two methods including DC resistivity logging and induction logging, in which the DC resistivity logging is to supply a DC current to the ground in a well or near a well wall and measure the potential difference to obtain the formation resistivity within a certain offset distance; the induction logging is based on the principle of electromagnetic induction, which supplies an AC current in a transmitting coil to generate an AC magnetic field excitation and obtains formation conductivity by measuring the second field produced by an eddy current within a certain offset distance.
In recent years, along with the development of information technology, domestic and international geophysical communities have researched and developed a crosshole electromagnetic tomography (CT) technique for measurement and imaging of crosshole resistivity distribution. Compared with conventional electrical logging, it has a deeper and broader detection range and compared with ground electromagnetic method, it has higher accuracy and resolution without being affected by a well depth. One of the challenges facing the borehole electromagnetic wave CT is conflict between detection range and resolution. The formation suitable for oil and gas reservoir is generally a strong loss medium for electromagnetic wave. If we intend to increase the detection range, transmission power must be increased and transmission frequency must be reduced. However, decreasing of the frequency will affect the resolution. In the application of oil field development, intensity attenuation of metal casing is also a problem. Theoretical studies show that, when transmitting with a vertical magnetic dipole source in a cased well and receiving in another borehole, if the transmission source frequency is of hundreds of hertz, a signal can penetrate the casing to radiate the formation. However when wells are both cased ones, signal detection is almost impossible. When a casing collar is in insulation connection, each section of the casing can be used as a transmitting antenna with a better effect, but it is unachievable in actual constructions of well drilling.
As to formation resistivity measurement in oil reservoir development, the main challenge is still the strong shielding effects of metal casing on electromagnetic field, making it difficult for measurements of either DC or induction field. With development of electronic technology and improvement of weak signal detection technology, foreign scholars have studied and achieved to supply power for casing walls and measure ultra weak gradient electric potential signal on casing walls with a view to obtain the formation resistivity information of outer cased formation by converting to a DC resistivity curve. Several logging technology service companies in USA such as Schlumberger, Haliburton, and Baker Atlas, have developed a through-casing resistivity logging instrument based on DC voltage potential measurement. This kind of through-casing resistivity logging method, which is affected by many factors in practical applications, makes it difficult to be popularized. First of all, because power supply and measurement must be implemented very close to well walls, casing walls need to be cleaned before measurement. Such cleaning work has high cost. If the cleaning work is not thorough, it will cause a great measurement error due to poor contact. This problem is caused by limitations of observation methodology. Measurement by direct contact with casing walls is more susceptible to the impacts of casing collars and perforating points, leading to difficulties in data interpretation. Second, because such method uses direct current power supply which is diffused into the different layer of formations through the casing and the DC diffusion field itself has no depth sounding ability, explorers have to rely on changes of electrode distance to distinguish the formation resistivity in layer of formations. Finally, due to the restriction of logging cables, supply current will not be high enough (<6 A). Thus, such method is very limited to detect depth. Consequently, as cement sheath have important impacts on observed data, it needs fine processing and correction in the interpretation, and inner reservoir resistivity distribution information within a certain distance from the well wall cannot be obtained at all.